A technology has been conventionally developed that performs communications, such as peer-to-peer, directly between information-processing devices, not via a server. In such communications, as shown in FIG. 9, information-processing devices PC1 and PC2 may communicate through NAT1 and NAT2, respectively. Here, NAT1 and NAT2 convert mutually between a private IP address at the local side (information-processing device) and a global IP address at the communication line side. NAT1, NAT2, etc. include a port allocation rule and intercepting filter rule, and NATs are classified according to a combination of these rules. Specifically, the transmission port allocation rule has three types. One is Cone type, where, if an IP address identifying a specific information-processing device is allocated to a port at the local (e.g. LAN) side of a NAT, a port at the global side (e.g. WAN such as the Internet), allocated to the NAT corresponding to the port at the local side, is also specified, independently of the destination (the IP address and port number) of a packet. Another is Address Sensitive type, where a new port number is allocated for each destination address of a packet. The other is Port Sensitive type, where a new port number is allocated for each destination port of a packet. Meanwhile, the intercepting filter rule, which judges if a packet can be received at the global side through the port where the packet has been transmitted from the local side of the NAT, has three types. One is Address Sensitive filter, where a packet is received only through the address with which the packet has been transmitted from the port. Another is Port Sensitive filter, where a packet is received only through a port through which the packet has been transmitted. The other is No filter, where a filter does not exist, but all packets are received.
In such communications via a NAT, if packets are not transmitted from the LAN side to the WAN side of the NAT, packets from other information-processing devices at the WAN side cannot be accepted. If the intercepting filter rule for the NAT particularly uses Address Sensitive filter or Port Sensitive filter, unless packets have not been transmitted to an information-processing device at the WAN side or to a given port of the information-processing device, packets transmitted from the information-processing device at the WAN side can not be accepted. Therefore, in order to establish communications between information-processing devices via the NAT, transmitting packets (bubble packet) for leaving a transmission history and other operations have been made.
These technologies are described in “STUN-Simple Traversal of User Datagram Protocol (UDP) Through Network Address Translators (NATs)” by J. Rosenberg, J. Weinberger, C. Huitema, and R. Mahy, online: March, 2003, retrieved: Mar. 17, 2004, URL: ietf.org/rfc/rfc3489.txt; and “Advanced Network Pack for Windows (registered trademark) explained Part 1 Teredo enables more transparent IPv6 connection” by Takuya Oikawa, online: Sep. 29, 2003, retrieved: Mar. 17, 2004, URL: ipv6style.jp/jp/tryout/20030929/2.shtml.
However, some types of NATs regard as an offensive a packet rejected due to an intercepting filter rule, and refuse a packet transmitted from the IP address for the communication source of the packet, to improve security level. In such a case, as a result of transmitting a bubble packet to the destination NAT, the bubble packet is regarded as an offensive. Consequently, the NAT that has received the bubble packet results in refusing any packet from the information-processing device that has transmitted the bubble packet, and may fails to appropriately establish communications between the information-processing devices at the source and destination of the bubble packet.
Further, a bubble packet is transmitted for leaving a transmission history in the source NAT so that it can appropriately receive a packet transmitted from the destination. Therefore, a bubble packet is not usually transmitted for transmitting information. It is not desirable that such packets, not for transmitting information, increase communication traffic on communication lines such as the Internet.